This invention relates to methods of making telechelic oligomers, methods of making block copolymers, and compounds useful in such methods.
The development of methods for synthesizing telechelic oligomers is an increasingly important area of research, as these materials have useful properties and may serve, for example, as foundations for the synthesis of block copolymers. See generally, Telechelic Polymers: Synthesis and Applications, (E. J. Goethals, ed., (CRC Press, Boca Raton, Fla., 1989); V. Percec et al., Macromonomers, Oligomers, and Telechelic Polymers in Comprehensive Polymer Science 6, 282-357 (G. C. Eastmon et al., eds., Pergamon Press, New York, 1989). Whereas a number of telechelic oligomers based on vinyl monomers have been reported, few polycarbonate (PC) polycondensation telechelic oligomers are known, despite the industrial importance of polycarbonates such as bisphenol A polycarbonate. See, e.g., C. O. Mork et al., J. Appl. Polym. Sci. 45,2289-2301 (1992) and J. S. Riffle et al., J. Polym. Sci., Polym. Chem. Div., 20, 2289-2301 (1982) (hydroxy-terminated telechelic PC); M. J. Marks et al., J. Polym. Sci., Polym. Chem. Div., 35, 385-390 (1994) and M. J. Marks et al., Macromolecules 27, 4106-4113 (1994) (non-hydroxy terminated telechelic PCs).
Two general methods for the synthesis of telechelic polycondensates are known: (1) the use of a stoichiometric imbalance of difunctional monomers during polymerization, including the addition of monofunctional monomers; and (2) the depolymerization of polymers using reactive small molecules (e.g., glycolysis). See, Polymeric Materials Encyclopedia, 7412-7414, J. Salamone, ed., (CRC Press, Inc., Boca Raton Fla., 1996). The former method has successfully yielded the known telechelic PCs, while the second method has had only limited success in producing telechelic PCs. See C. H. Bailly, J. Polym. Sci. (Polym. Phys. Div.) 23, 493 (1985). Additionally, several examples have been reported for methods of breaking down high molecular weight polyesters (e.g., in methods of recycling) by adding a significant excess of alcohol or water to the polyester to gain the starting monomers (i.e., diacid, diol, their derivatives or low molecular weight adducts). Using similar methodology, polycarbonates may be recycled by adding water to yield a bisphenol. In general, a transesterification catalyst, heat and optionally a solvent is required for this method. Transesterification catalysts reported include acids, bases, and metal-organic compounds based on metals such as tin, titanium, magnesium, calcium, or zinc.
Telechelic oligomers may be used to carry out three important operations: (1) the formation of linear and branched long polymer chains by the chain extension of short polymer chains; (2) the formation of networks; and (3) the formation of block copolymers. See, e.g., Polym. Prepr. 38(2), 695 (1997). Block copolymers often have properties that are unavailable in homopolymers or in mixtures of homopolymers. Block copolymers may be useful in the formation of, e.g., thermoplastic elastomers (TPE), where materials contain xe2x80x9chardxe2x80x9d and xe2x80x9csoftxe2x80x9d segments that phase-separate to give rise to elastomeric behavior. The xe2x80x9chardxe2x80x9d segments act as reversible crosslinks during thermal treatment, such treatment causing a transition to the thermoplastic. Block copolymers are additionally useful in polymer compatibilization methods, where small amounts of block copolymers reduce surface energy and increase the surface adhesion of two normally imiscible polymer phases, resulting in blends with superior properties as compared to uncompatibilized, macrophase-separated polymers. Block copolymers may also serve as surfactants in various processes.
Two known methods for synthesizing block copolymers involve the utilization of living polymerization methodologies and telechelic oligomers. Living methods allow stable propagating end groups to insert a second monomer, thereby accessing AB-type blocks. In principle, this methodology should access many polymer types. However, the living technology is typically only accessible to chain-growth processes, thereby eliminating materials exclusively synthesized via step-growth-type mechanisms. Additionally, the kind of xe2x80x9cBxe2x80x9d monomers compatible with known xe2x80x9cAxe2x80x9d monomers is limited by the choice of xe2x80x9cAxe2x80x9d monomers.
An alternative method of synthesizing block copolymers involves the use of telechelic oligomers, which through the presence of reactive end groups allows new polymer chains to be grown or grafted. Polymers that are typically synthesized using step-growth type processes are amenable to this approach through the utilization of comonomers that effectively cap the growing polymer chain. However, few general methods have been reported for the synthesis of telechelic polyesters and polycarbonates using this approach. As a result, the properties of such important industrial polymers have not been widely expanded by their incorporation into well-defined block materials.
Accordingly, a need exists for general and convergent techniques for the construction of di- and multi-block polymers such as polyesters and polycarbonates. Such techniques would allow for the incorporation of numerous and functionally diverse reactive end groups into a wide variety of polyester and polycarbonate oligomers. Telechelic oligomers produced by such methods may serve as building blocks for the convergent construction of, for example, di- and triblock polyester and polycarbonate copolymers.
The present invention is based upon the discovery that high molecular weight polymers (e.g., polyesters and polycarbonates) may be broken down in the presence of a chain transfer agent (CTA) and catalytic amounts of alkali metal alkoxides, producing telechelic oligomers in a relatively short period of time. It has additionally been discovered that the ring-opening of cyclic esters in the presence of acyclic esters may yield ester-bearing telechelics, thus generally providing for 100% endgroup control.
Accordingly, one aspect of the invention is a method of making a telechelic oligomer, comprising reacting a substrate compound selected from the group consisting of polycarbonates, polyesters, polyurethanes, polyarylates, cyclic esters, cyclic carbonates, and cyclic urethanes with a chain transfer agent (CTA) in the presence of an alkali metal catalyst to form a telechelic oligomer. Telechelic oligomers produced by such a method may then be further reacted with monomers according to known polymerization methods in order to produce block copolymers. Another aspect of the invention is a method of producing a block copolymer comprising reacting a substrate compound as provided herein with a chain transfer agents comprising a polymeric residue in the presence of a catalyst as provided herein, thus producing a block copolymer. An alternative embodiment of this aspect of the invention comprises reacting a substrate compound comprising a polymeric residue with a chain transfer agent as provided herein, in the presence of a catalyst as provided herein, in order to yield a block copolymer. Additional aspects of the invention include telechelic oligomers and block copolymers prepared by methods of the present invention, as well as certain novel block copolymers.
These and other aspects of the invention are set forth in the detailed description of the invention below.